The manufacture of seamless cellulose tubes for sausage casings using cellulose derived by the so-called "viscose process" is well known in the art. Briefly, in the viscose process, a natural cellulose such as cotton linters or wood pulp is reacted chemically to form a cellulose derivative (cellulose xanthate) which is soluble in a weak caustic solution. The solution or "viscose" is extruded as a tube into an acid bath. The extruded viscose reacts chemically with the acid bath resulting in the regeneration and coagulation of a pure cellulose tube. The chemical reaction produces several by-products including hydrogen sulfide and carbon disulfide.
More recently, a process of direct cellulose dissolution has been adapted to the manufacture of cellulose food casings. In this process no cellulose derivative is formed so the chemical reactions required first to form a cellulose derivative and then to regenerate the cellulose from the derivative have been eliminated. Instead, a natural cellulose is put directly into solution with the use of a tertiary amine oxide cellulose solvent such as N-methyl-morpholine-N-oxide (NMMO). The resulting solution is thermoplastic in that it hardens upon cooling and flows on reheating. The solution, when molten, can be extruded into a water bath. The NMMO solvent is extracted in the water bath so that a regeneration of the cellulose solution occurs. Use of NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose, as in the viscose process. This avoids certain disadvantages of the viscose process such as the generation of gaseous sulfur compounds during the regeneration process.
U.S. Pat. Nos. 2,179,181; 4,145,532; 4,426,228 and Canadian Pat. No. 1,171,615 all deal with the formation of a cellulose solution using the NMMO solvent and subsequent formation of cellulose articles such as films and filaments using the resulting solution. An apparatus and method for preparing an extrudable cellulose solution in a continuous process are disclosed in U.S. Pat. Nos. 5,094,690 and 5,330,567. In these patents, a suspension of cellulose in an aqueous solution of NMMO is fed into the top of a vessel having a heated wall. Within the vessel, a rotating wiper spreads the suspension across the heated wall and moves the suspension downward in the vessel. As the suspension moves downward, water is evaporated and the concentration of NMMO increases. Eventually, the temperature of the suspension and the concentration of NMMO reaches a level where the cellulose is dissolved so that a cellulose solution flows from the bottom of the vessel.
U.S. Pat. Nos. 5,277,857; 5,451,364 and 5,597,587 disclose a tubular extrusion method and apparatus utilizing the thermoplastic cellulose solution for purposes of making tubular films. Such films, for example, may be used as sausage casings.
As disclosed in these patents, the cellulose solution is extruded through an annular die and into a bath of nonsolvent liquid. This can be, for example, water but a nonsolvent concentration of water and NMMO is preferred. This is because one advantage of this direct cellulose dissolution method for producing a cellulose film is that the solvent is recoverable and can be reused. This adds to the economy of the method in that the solvent extracted during cellulose regeneration can be recycled into the system to dissolve the natural cellulose for extrusion. Thus, while water alone is preferred from the standpoint of the speed of cellulose regeneration, an initial higher concentration of NMMO in the bath renders the recovery of the solvent for reuse more cost effective. For example, a regenerating bath having an initial concentration of at least 10% wt. NMMO is considered cost effective for solvent recovery with a range of 15-50% wt. being preferred.
A method for recovering the NMMO is disclosed in WO 93/11287. Briefly the method involves a step of purifying the bath liquid (water+NMMO) by treatment with a strongly basic anion-exchange resin wherein the resin is regenerated by successive treatments with an aqueous solution of a strong inorganic acid and an aqueous solution of sodium hydroxide.
For purposes of forming tubular films as disclosed in the above referenced patents, the extrusion occurs about a mandrel which depends from the die. An accommodation is made for the introduction of liquid from the regenerating bath into the volume inside the extruded tube. This introduction of the regenerating liquid into the interior of the extruded tube is said to perform several functions. These include, for example, lubrication of the mandrel to facilitate passage of the extruded tube and initiation of the regeneration of cellulose at the inner surface of the extruded tube.
For purposes of size control of the extruded tube, it is preferred that the mandrel have a section which is larger in diameter than the extruded tube so that the tube is diametrically expanded as it passes over the mandrel. However it has been found that when the mandrel diameter is larger than the extruded tube diameter, water alone is not a sufficient lubricant.
Introducing a lubricant such as an oil or the like into the interior of the extruded tube was not attempted because this would add a third component (the lubricant) which would complicate the recovery of the NMMO.
Accordingly, a solution of water and NMMO has been preferred for introduction into the extruded tube. The solution for such introduction can be drawn directly from the regenerating bath (the "outer bath") and introduced into the extruded tube through the mandrel as disclosed, for example, in U.S. Pat. No. 5,277,857. To insure that the extruded tube does not bind on the mandrel, a higher minimum concentration of 30% NMMO solvent is useful with a preferred range being 30-50% as disclosed in U.S. Pat. No. 5,451,364. Thus, an operable system can use a 15% NMMO concentration for the outer bath liquid and a 30% NMMO concentration for the liquid introduced into the extruded tube. In subsequent stages of the manufacturing operation these liquids become mixed for extraction and reuse of the solvent.
For process purposes, it is desirable to regenerate the cellulose from the extruded tube as quickly as possible. For example, it is known that a rapid regeneration produces a more dense cellulose structure which in turn enhances the strength of the cellulose film which is produced. This desire for rapid regeneration is offset by the need for introducing more lubricant into the interior of the extruded tube and the desire for efficient solvent recovery. Accordingly, as noted above, an NMMO solution has been used for both liquids instead of water alone. It now has been discovered that the solvent can be eliminated entirely from the liquid introduced into the interior of the extruded tube without sacrificing lubricating properties to prevent binding of the extruded tube to the mandrel. The elimination of solvent has the added advantage that the regeneration of cellulose at the inner surface of the extruded tube is accelerated resulting in a denser cellulose structure and enhanced properties.
It has been found that the addition of a suitable surfactant, preferably a nonionic poly(ethylene oxide) rather than NMMO or other lubricant, will provide sufficient lubricity to prevent binding of the extruded tube to the mandrel. Thus, the surfactant addition eliminates the need for having NMMO solvent in the liquid introduced into the interior of the extruded tube (thereby providing the advantage of speeding regeneration) while providing the necessary lubrication to prevent binding.
The liquid introduced into the interior of the extruded tube eventually is drawn into a volume within the extruded tube below the mandrel. This volume or "inner bath" along with water and surfactant, contains an amount of NMMO solvent extracted from the extruded tube. Liquid removed from the inner bath as disclosed in U.S. Pat. No. 5,277,857 can be mixed with liquid from the outer bath for solvent recovery. The presence of the surfactant in the mixture of the inner and outer baths was found not to interfere with solvent recovery. This is because the bath mixture, prior to solvent recovery, is subjected to an anion exchange column. The anion column which is designed to remove certain by-products of cellulose dissolution and regeneration also removes the nonionic poly(ethylene oxide) surfactant which then is destroyed during the acid regeneration of the column. Thus, it has been found that the use of a nonionic poly(ethylene oxide) surfactant does not introduce a third component that complicates recovery of solvent from the bath.
Accordingly, it is an object of the present invention to provide a process for regenerating a seamless cellulose tube (suitable for use as a sausage casing) from a solution of nonderivatized cellulose, a tertiary amine oxide cellulose solvent and water in which regeneration of the cellulose at the inner surface of the extruded tube is accelerated.
Another object is to provide such a process in which the regeneration of cellulose at the inner surface is accelerated by introducing a solvent-free regenerating liquid into the interior of the extruded tube.
A further object is to provide such a process wherein the extrusion occurs about a mandrel which diametrically expands the extruded tube and the solvent-free regenerating liquid introduced into the interior of the extruded tube is an aqueous solution containing a lubricant to prevent the binding of the extruded tube to the mandrel.
Yet another object is to provide such a process wherein the lubricant which is a component of the regenerating solution does not interfere with the subsequent purification and recovery of NMMO from the regenerating liquids.